But if you just thought “Life?!” think again. Despite the presence of water molecules in their atmospheres, none of these five planets is suitable to life as we know it. They are all gas giants, and they are all too close to their suns to sustain liquid water on their surfaces.

These planets -- WASP-17b, HD209458b, WASP-12b, WASP-19b and XO-1b -- are tidally locked with their stars so that the same side of the planet always faces the star and is constantly bombarded with a powerful stream of solar radiation. The heat from the star is distributed around the planet via strong winds that move faster than the speed of sound.

“They are very violent places,” said L. Drake Deming, an astronomer at the University of Maryland who recently led a census of exoplanet atmospheres.

To find traces of water vapor in their atmospheres, Deming and his colleagues used Hubble’s Wide Field Camera 3, which measures near-infrared light. The scientists waited until each planet was about to pass in front of its sun, then pointed Hubble at it. Because water molecules absorb certain wavelengths of light, the scientists could detect the presence of water vapor during the planet’s transit across its sun if those wavelengths were not picked up by Hubble.

Deming said that finding water vapor on these planets was not a surprise. Indeed, he would have been surprised if he hadn’t found it.

“It was consistent with what we expect,” he said. “But it is one thing to say these planets should have water vapor and it’s another thing to actually measure it.”

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Hubble is only strong enough to see the signs of water vapor on the gas giant planets, but Deming said that Hubble’s successor -- the James Webb Telescope, slated to launch in 2018 -- will be strong enough to find water signatures in the atmospheres of smaller, rockier planets more like Earth.

“The James Webb Telescope would allow us to extend these measurements to habitable Earth-like planets, maybe twice as big as Earth, and in the habitable zone where water could condense,” he said.

In the meantime, Deming and his team want to determine the most accurate way to measure the presence of water on planets trillions of miles away, and the hot Jupiter planets make good test cases. Because they are so close to their host stars, most of them take just days to complete a full orbit, offering scientists frequent opportunities to take measurements.

“It wouldn’t be a bad analogy to say these planets are the astronomical equivalent of fruit flies,” he said."The grand challenge is to find another habitable Earth-like planet, but the focus of my work is to put these measurements on a really sound basis.”

If you love thinking of hot Jupiters as the fruit flies of the universe, then we have a lot in common. Follow me on Twitter for more like this.